Hurricane Sandy produced 11-m high waves in 66-m water depth on the continental shelf south of Cape Cod and 5-m high waves in 12-m depth 2 km offshore of the southern shore of Martha's Vineyard, MA. The large waves combined with strong tidal- and wave-driven currents can transport tremendous amounts of sediment along the shoreline, and into the inlet during flood tides. Visual inspection of Katama Bay and Inlet after Hurricane Sandy suggest there were large changes to the ebb shoal, shoreline, inlet, and back bay. A post-Sandy survey of the ebb shoal, inlet channel, and back bay near Katama Inlet will be conducted. When combined with the pre-Sandy survey and observations of tides, waves, and currents that were collected by the investigators before, during, and after Hurricane Sandy, the proposed observations will allow the data and numerical models to be used for addressing the following hypotheses:
(i) the timing of storms relative to ebb and flood flows is important to the morphological evolution, and
(ii) accretion inside the bay is caused by sand carried alongshore from the west and subsequently transported into the inlet during flood flows.
An additional hypothesis is that wave radiation stresses drive water into the inlet, producing stronger flood and weaker ebb flows, further enhancing the transport of sediment into the inlet during storms.
Broader Impacts: The project investigators have been providing the local harbormaster, boating organizations, and shell fishermen with updated bathymetric maps to help them with their operations. Improved understanding of, and numerical models for, the processes affecting the morphology would help them with future management decisions as the inlet evolves. For example, the US Army Corps of Engineers has investigated establishing a sustainable breach between Katama Bay and the Atlantic to prevent siltation of the commercial oyster farms in the bay, potentially resulting in several million dollars per year of benefit to the local shell fishing industry. In addition, the results from this study will help harbormasters plan operations as the inlet evolves and currents change in Edgartown Harbor. Moreover, a field-verified modeling system of inlet evolution can be used to simulate a range of nearshore systems, allowing simulations and what-if scenarios in other areas.
In addition, the observations will be used by three WHOI-MIT PhD students as part of their dissertation research about the effects of waves on shoreline morphological evolution and on inlet flows.
The primary outcome of this project is the documentation of the changes to the shoreline near Katama Inlet, Katama Bay, and the southern shore of Martha's Vineyard caused by Hurricane Sandy. During the evening of Oct 29, 2012 wave heights from Hurricane Sandy exceeded 11 m in 66-m water depth on the continental shelf south of Cape Cod and were 5 m in 12-m depth, 2 km offshore of the southern shore of Martha's Vineyard, MA. Surface currents driven by tides and winds in 12-m depth were almost 1.5 m/s. The combination of these flows with breaking-wave induced alongshore flows in the surfzone produced strong currents that carried sediment along the straight sandy coastline on the southern shore of Martha's Vineyard (Figure 1A). However, the alongshore transport of sediment is interrupted by the tidal currents flowing into and out of Katama Bay via Katama Inlet. Consequently, the sediment carried along the shoreline can be swept into the inlet and deposited in Katama bay during flood tides. We measured more than 2.5 m vertical elevation changes of the seafloor near and within Katama Inlet caused by Hurricanes Irene (Figure 1C) and Sandy (Figure 1D). In both storms (in which the largest waves coincided with peak flood flows into the inlet), sediment was transported toward and into the inlet by tide- and wave-driven currents, filling in the main inlet channel (yellow-red contours in Figure 1C and D along the bay side of Norton Point and crossing over towards Chappaquiddick Island). During Irenethe inlet channel migrated tens of meters to the east in less than 36 hours, and a tremendous amount of sand was moved into the bay behind the inlet (not shown, north of the regions in Figure 1C and D), burying large sections of commercial oyster farms. Over the next 6 weeks the newly deposited sand in the bay moved back toward the inlet, and then offshore in the strong ebb flows in the inlet channel. Hurricane Sandy caused similar bathymetric changes (Figure 1D), with the inlet channel migrating farther east (blue contours in Figure 1D adjacent to Chappaquiddick Island) until abutting the banks of Chappaquiddick Island. The observations of the shoreline change caused by Hurricane Sandy gathered in this project are being used by graduate students Julia Hopkins and Mara Orescanin for their thesis research about waves, currents, and sediment transport near Katama Inlet. In addition, two undergraduate student fellows will use the data for thier research projects in summer 2013.
